Answer

By Your Horse

Natural Horsemanship Expert Charles Wilson advises:

There’s still so much we don’t understand about equine behaviour. The dynamics of herd behaviour are governed principally by body language – these ‘conversations’ can be as subtle as the flick of an ear, or expressed as energetic biting, bullying, kicking and fighting.
The situation might improve with time – as the horses have been put together, there will be challenges between them to see who’s the leader. This leadership struggle, or pecking order, is maintained in a subtle way in a stable, close-knit herd, but three geldings, who are strangers to each other, will have to sort things out with rather cruder behaviour. However, in the wild, male horses live in ‘bachelor herds’ if they haven’t managed to steal their own mares, so it’s not totally against nature to have male horses together.If your gelding doesn’t settle and continues to be aggressive with other horses, it might be worth checking if he’s a rig.
A rig is a horse who has not been gelded properly, usually due to undescended testes. In general, it’s best to introduce horses singly to a herd, meeting each other over a fence before turning them out into a field together. Back shoes are best removed for a week or two to prevent nasty injuries from kicks. Finally, never try to separate two fighting horses by getting in between them – stay out of the field until things have settled down.